The use of high purity 2,2-bis(4-hydroxyphenyl)propane, herein referred to as bisphenol-A, as a reactant in the preparation of subsequent formulations such as the preparation of polycarbonate resins is well known in the art. One method for obtaining bisphenol-A of the purity needed is to distill crude bisphenol-A. In such distillations, various by-products remain and these primarily include higher condensation products of bisphenol-A, condensation products of phenol and acetone produced in the original bisphenol-A formation, colored substances, isomers of bisphenol-A, and the like. Prahl et al, U.S. Pat. No. 3,290,390, disclose the addition of phenol to the by-products and contacting the resulting mixture with an acidic agent such as hydrogen chloride at between room temperature and 150.degree. C. to produce therefrom bisphenol-A. However, the method of Prahl et al requires, according to their examples, 16 hours or more reaction time.
It has now been discovered that it is possible to convert such by-products isomerizable to bisphenol-A to said bisphenol-A in a vastly shorter period of time by treating a mixture of the by-products and phenol with anhydrous hydrogen bromide. The process of this invention yields bisphenol-A in solution for a subsequent recovery such as by cooling to produce a 1:1 bisphenol-A/phenol adduct, from which the bisphenol-A can then be recovered, by procedures known per se.
In practice, commercial plants utilize distillation of bisphenol-A as a step in the purification process. The bottoms from the distillation are called "tars" and conventionally these are disposed of by burning. This represents a serious loss in yield because tars contain from 20-60% bisphenol-A plus the isomerizable by-products mentioned above. Experiments have shown that the amount of bisphenol-A in a typical tar can be increased from 30% to 60% by isomerization with anhydrous HBr and more than half of this bisphenol-A can be recovered, e.g., by crystallization. The mother liquor from the crystallization can be stripped of phenol (for recycle) in a separate column, and the diminished quantity of "tar" remaining can be burned or otherwise utilized.